The present invention relates to a method of driving a matrix display device of the type in which each matrix element comprises a series-connected combination of a non-linear element and a display element such as a liquid crystal display element. More specifically, the present invention relates to a drive method whereby non-linear elements having a low value of threshold potential can be used, while maintaining a sufficiently large operating margin to allow for manufacturing deviations in element characteristics. Various types of matrix display devices, utilizing a liquid crystal, electro-chromic or other types of display element, have now reached the stage of practical application, and methods are now being considered for producing high-density matrix displays. In general, the most satisfactory drive method which has been developed until now for such displays has been the "active matrix" method, in which active elements (e.g. thin-film FET transistors) are employed to control the display elements, with one active element being provided for each display element and formed on a display panel closely adjacent to the corresponding display element. This active matrix drive method is satisfactory from the aspect of providing a sufficiently high tolerance against the effects of stray deviations in the characteristics of the display elements and the active elements themselves to ensure reliable operation. Such an active matrix display, utilizing transistors as control elements, has been described for example by B. J Leichner et al in a report published in the Proceedings of the IEEE, volume 59, No. 11, pages 1566 to 1579.
However it is desirable to simplify the configuration of all of the elements of a matrix display as far as possible, in order to ensure maximum manufacturing yield, and to maximize the available display area as far as possible, both by minimizing the area occupied by the control elements and that occupied by connecting leads coupled to these control elements (e.g. the connecting leads to the gate electrodes of transistors used as control elements). For this reason, it has been proposed to utilize a "active matrix" type of display, in which active elements, i.e. 2-terminal devices having a suitably non-linear voltage/resistance characteristic, are used as control elements. Such a method has also been described by Leichner et al in the above reference. It has been proposed to use ceramic varistors as such non-linear control elements, for example as described by D. E. Castleberry in the IEEE ED-26, 1979, pages 1123 to 1128. In addition, it has been proposed to use MIM type diodes for such non-linear elements, for example as described by D. R. Baraff et al, in the IEEE ED-28, 1981, pages 736 to 739.
However, various problems have arisen with such prior art proposals for utilizing active elements. These are:
1. Lack of uniformity in element characteristics.
2. Operation of the display can be strongly affected by the distribution pattern of element characteristics, and by stray deviations in these characteristics.
3. The threshold potential Vth of the elements must be high.
4. The drive voltage levels required are high.
The most serious disadvantage of these prior art proposals has been that it is necessary to use active elements having a high level of threshold voltage, in spite of the fact that some types of display element such as liquid crystal display elements can operate at drive voltage levels as low as two or three volts. Elements having an inherently high value of threshold voltage, such as varistors or zener diodes, are not suited to formation on display matrix panels, e.g. in the form of thin-film elements closely adjacent to display elements, and in addition elements such as varistors have a considerable stray deviation in their threshold voltage values. In addition, it is desirable that such a matrix display device can be operated from a low value of supply voltage, to facilitate use in portable equipment, so that a requirement for high drive voltage levels is a significant disadvantage.
With the present invention, a new drive method is employed, whereby the disadvantages 2, 3 and 4 above are considerably reduced, and whereby it becomes possible to use elements which have a low value of threshold potential, so that disadvantage 4 is substantially eliminated. Thus, the method according to the present invention enables the threshold voltage (in the forward conduction direction) of a single PN diode to be utilized to control each display element, so that a suitable non-linear element can be configured as a pair of PN diodes connected in parallel with opposing polarities. Such diodes have a much higher degree of stability and uniformity of characteristics than the devices such as MIM diodes or varistors which have been previously proposed for use an non-linear elements in matrix display devices. In this way, the present invention considerably reduces problem (1) above, and brings such displays significantly closer to the stage of practical application.